The continually increasing combustion of fossil fuel, such as coal, natural gas and oil, during the last centuries has resulted in an increase in the concentration of CO2 in the atmosphere. The increasing concentration of CO2 has caused concern due to the greenhouse effect caused by CO2. The greenhouse effect is suspected already to have caused at least some of the changes in the climate that have been seen during the last decades, and is according to simulation models suspected to cause even more and potentially dramatic changes in the climate of planet earth.
This has caused a call for action from scientists, environmentalists and politicians throughout the world, to stabilize or even reduce the discharge of CO2 from combustion of fossil fuel to the atmosphere. This may be achieved by capturing and safe depositing of CO2 from the exhaust gas from thermal power plants and other plants where fossil fuel is combusted.
The captured CO2 may be injected in sub terrain formations such as aquifers, oil wells for enhanced oil recovery or in depleted oil and gas wells for deposition. Tests indicate that CO2 remains in the sub terrain formation for thousands of years and is not released into the atmosphere.
Capturing of CO2 from a gas by means of absorption is well known and has been used for decades, e.g. for removal of CO2 (and other acid gases) from produced natural gas at gas fields. The absorbents used or suggested in the prior art have been different aqueous alkaline solutions, such as potassium carbonate, see e.g. U.S. Pat. No. 5,528,811, and different amines, see e.g. U.S. Pat. No. 4,112,051, U.S. Pat. No. 4,397,660 and U.S. Pat. No. 5,061,465. Separation of CO2 from exhaust gas from thermal power plants by means of an amine solution, is known e.g. from U.S. Pat. No. 4,942,734.
Common for these CO2 capturing solution is that the gas mixture to be separated is introduced countercurrent to the aqueous adsorbent in an absorber column. The gas leaving the absorber column is CO2 depleted (or acid gas depleted), whereas the CO2 (or other acid gas) leaves the absorber column together with the absorbent. The absorbent is regenerated in the regenerator column and returned to the absorber column. Amine is regenerated by stripping the amine solution with steam in the regeneration column. The steam is generated in the reboiler at the base of the column.
As illustrated above CO2 as such is well known in the art. However, there is a need for several improvements in the CO2 capturing process to make CO2 free or low CO2 emission thermal power plants economically profitable.
The plants for capturing of CO2 are relative large, complex and expensive constructions. It is therefore desired to reduce the size, complexity and cost of the Plants.
Capturing of CO2 is carried out at the expense of the efficiency of a thermoelectric power plant utilizing fossil fuel, so that the output of electrical power and/or medium temperature heat from a thermoelectric power plant is reduced. The reduced efficiency compared with a traditional plant makes these facilities less profitable. Improvements in the efficiency, i.e. reducing the energy cost in the CO2 capturing process, are therefore sought.
The currently preferred absorbents are aqueous solutions of different amines. The commonly used amines are alkanol amines, such as e.g., diethanol amine, mono methyl ethanolamine, aminoethyl ethanolamine, 2-(Methylamino)etanol, MDEA as well as other amines known by skilled man in the art. The absorption of CO2 to the amine absorbents is a reversible, exothermic reaction. Accordingly, heat has to be supplied to the regenerator column to reverse the absorption and release the CO2.
The amine absorbents are exposed for degradation during the absorption/desorption cycle described above. The exhaust gas from the combustion of carbonaceous material comprises oxygen which will react with the amines dependent on the type of amine, the oxygen concentration, the total contact time between oxygen and amine, temperature etc. High temperature, long contact time and high oxygen content in the gas to be treated, are factors that increase the degradation of the amine. The degradation of the amine is also a factor adding cost to the capture of CO2. The degraded amines have to be replaced and deposited resulting in increased operational costs of the plant.
The exhaust outlet temperature from a traditional thermal power plant is normally 80 to 120° C. as the gas at lower temperatures has no value in production of electrical power, and no or limited value as a heat source in other processes.
A cooler is therefore introduced between the outlet of a thermal power plant and the absorber column. The most common cooler is a tower in which the exhaust gas is cooled by a countercurrent flow of water spray both to cool the gas and saturate the gas with water vapor. The cooled and saturated gas leaving the cooler has normally a temperature of about 50° C. and is directly introduced into the absorption column.
The cooler and absorber for a CO2 capturing plant, constitutes a substantial part of the total investments for the plant. A decrease of the size and complexity and thus the total cost of investments for the cooler and absorber is therefore sought. Additionally, there are significant operating costs connected to the cooler. There is a pressure loss across the cooler resulting in increased power consumption. Additionally, the cooler requires large power consuming pumps.
An objective of the present invention is thus to provide solutions that makes it possible reduce to cost of investments for the cooler and absorber in a CO2 capturing plant, without an unacceptable increase in operational costs and/or decrease in efficiency of the plant.